1. Field of the Invention
This invention relates to a constant phase gain control circuit for use in conjunction with a dual gate field effect transistor (FET) in constant phase variable power amplifier applications.
2. Brief Description of the Prior Art
Phased array radar systems are being increasingly utilized in the military electronics industry. Many such systems are presently under development as evidenced by the articles "A Dual-Gate FET Constant Phase Variable Power Amplifier," of D. M. Drury, D. C. Zimmerman and D. E. Zimmerman, 1985 IEEE MTT-S International Microwave Symposium Digest, "Class-B Operation of Microwave FETs for Array Module Applications,", of M. Cohn, J. E. Degenford and R. G. Freitag, 1982 IEEE MTT-S International Microwave Symposium Digest and "High Efficiency Single-ended and Push-pull Class-B FET Power Amplifiers," of R. G. Freitag, J. E. Degenford and M. Cohn, 1985 Government Microcircuit Applications Conference. These systems require extremely small module packages. These systems require the extensive use of microwave monolithic integrated circuits (MMICs) as the active elements in the module. A particular element in these systems which has been the topic of much research is the constant phase variable power amplifier (VPA).
The approach taken in the above referenced class-B articles is the use of single gate FETs configured for class-B operation. The authors in the above noted Cohn et al. article have reported phase variations less than six degrees from a 15 db driver level control range. This approach appears to have merit except that it does not address the entire problem involved. No consideration is given for electronically controlling the drive level. This must be provided in order to achieve a taper in the array once the modules are integrated into a system.
Another approach which is presently under development in airborne phased array power amplifiers is the use of dual gate devices to implement power programmability. One inherent need in such applications is the ability to provide power programmability with a low phase error or phase change over the programmable region. One approach is a monolithic variable power amplifier (VPA) which uses dual gate FETs. The dual gate FETs are configured with an rf termination on the second gate of the dual gate FET, chosen for optimum phase performance on the second gate of the FET. Dual gate FETs configured in this way have been modelled as two single gate FETs connected in cascode. See, for example, the above noted Drury et al. reference as well as an article entitled "Microwave Wide-Band Model of GaAs Dual Gate MESFETs," of C. Tsironis and R. Meierer, IEEE Transactions, Microwave Theory Technology, Vol. MTT-30, pages 243 to 251. The variable power feayture is achieved by changing the bias applied to the second gate. By implementing the gain control feature in this fashion, the voltage standing wave ratio (VSWR) effect is eliminated at the input as a result of the reverse isolation of the "first FET" in the dual gate FET equivalent circuit.
There are several elements in these circuits, some nonlinear in nature, which are sensitive to process and fabrication variations. As a result of these process and fabrication variations, dual gate FET VPAs of the prior art have typically demonstrated transmission phase variations which are in excess of the system level requirements.
To date, dual gate variable power amplifiers have implemented the variable power mode by changing the bias applied to gate-2 of the dual gate FET exclusively as demonstrated in the above noted Drury et al. article. Since these devices operate in the saturated regime, there are elements in the FET equivalent circuit which are nonlinear in nature.